I read the article to mean that the bag-o-crete was on the shoulder, and that he hit it after being forced onto the shoulder by the semi's wake, wobble or something else. It was the bag that ultimately caused the crash, but not the events leading up to it. Clearly there is a lot that is unknown - at least to us.
As far as operator skill and knowing the bike's limits, motor officers are highly trained riders, typically able to do rather incredible things with those big heavy bikes they ride. Obviously I can't speak for the poor guy here, but I wouldn't assume that he didn't know the machine's "normal" limits. Maybe the combination of speed and aerodynamics initiated an unlikely, but very dangerous condition. Now whose fault that is...